Influence of mechanical properties on the nanowear of uniaxially oriented poly(ethylene terephthalate) film

Aligned regularly spaced ridges form immediately on scanning the surface of uniaxially oriented poly(ethylene terephthalate) (PET) at applied loads of greater than 15 nN. This contrasts with nanowear of biaxially oriented PET film studied previously [B.D. Beake, et al., Polymer 42 (2001) 7025], which proceeds more slowly over a number of scan cycles by fatigue wear. Differences in nanowear are interpreted in terms of the differences in crystallinity and mechanical properties with processing history. Nanoindentation shows that the biaxially oriented material is harder, higher modulus and has a higher H/E ratio. It is suggested that the stick-slip process that leads to the formation of regularly spaced ridges on the polymeric surface proceeds smoothly on the uniaxially oriented PET film as the yield threshold for plastic deformation is exceeded at these contact pressures. The higher crystallinity of the biaxially oriented material results in a higher yield stress thus imparting a barrier to the stick-slip process. It is necessary for some local damage to occur over a number of scan cycles before the surface has a low enough yield stress that the stick-slip process can proceed efficiently. Results are compared to the nanowear of other polymeric materials and a map of the major deformation processes in nanowear is proposed for the first time.